Ukraine's prime minister on Wednesday accused Russia of "exporting terrorism" to Ukraine by using covert forces to organise armed separatists who he said had attacked Ukrainian forces and occupied state buildings.

"The Russian government must immediately call off its intelligence-diversionary groups, condemn the terrorists and demand that they free the buildings," Arseny Yatseniuk told a government meeting.

"That is, if the Russian Federation is interested in stabilizing the situation, which I have significant doubts about," he added.

Earlier Reuters reported that six armored personnel carriers entered the eastern Ukrainian town of Slaviansk, with the lead vehicle bearing the Russian flag, an eyewitness said.

Several armed men, who wore different types of battle fatigues and appeared to be pro-Russia activists, sat atop each of the vehicles. The vehicles stopped outside the town's city hall which has for several days been occupied by separatists.

The second vehicle carried the emblem of the Donetsk People's Republic which a group of separatists proclaimed last week and which the Kiev authorities see as a first step to seeking to break from Ukraine and join with Russia.

The men on top of the troop carriers, some of whom were masked, were armed with Kalashnikov rifles, grenade launchers, knives and pistols.

At least three of the armored vehicles had been under the control of Ukrainian armed forces earlier on Wednesday, Reuters photographers said. A soldier manning one of the troop carriers now under the control of pro-Russian separatists identified himself to Reuters as being a member of Ukraine's 25th paratrooper division from Dnipropetrovsk.

He said: "All the soldiers and the officers are here. We are all boys who won't shoot our own people."

The Ukrainian Defense Ministry rejected reports that an army formation defected to join pro-Russian separatists are false. Defense Minister Mikhail Koval also said a state of emergency would not be declared in eastern Ukrainian regions.

The convoy came in from the direction of Kramatorsk, 15 kilometers (six miles) to the south, where Ukrainian airborne troops on Tuesday secured control over a military airfield.

The armed men waved to people as they drove into the town where separatists occupy several buildings including the police headquarters and the offices of the state security service.

Some people waved back and shouted "Well done lads !" People also chanted "Russia" Russia!"

There was no sign of Ukrainian troops in the city despite an announcement by Kiev authorities on Tuesday that a military-backed "anti-terrorist" operation was being extended to Slaviansk after the military action in Kramatorsk.

The origin of the troop carriers was not immediately clear.

NATO responds

In response to what NATO characterized as Russian aggression, the alliance decided to strengthen its military footprint along its eastern border immediately, the alliance's chief said Wednesday.

Secretary General Anders Fogh Rasmussen said NATO's air policing aircraft will fly more sorties over the Baltic region and allied ships will deploy to the Baltic Sea, the eastern Mediterranean and elsewhere if needed.

"We will have more planes in the air, mores ships on the water and more readiness on the land," Fogh Rasmussen told reporters in Brussels, declining to give exact troop figures.

Moscow must make clear "it doesn't support the violent actions of well-armed militias or pro-Russian separatists" in eastern Ukraine, he added.

NATO's eastern members — including Lithuania, Estonia, Latvia and Poland — have been wary following Russia's annexation of Ukraine's Crimean Peninsula, demanding a more robust military deterrence to counter neighboring Russia.

He said the NATO deployments are about "deterrence and de-escalation" in the face of Russia's aggressive behavior.

The NATO chief did not mention naval deployments to the Black Sea — which Russia would likely see as a direct aggression even though NATO members Bulgaria, Romania and Turkey also border the sea. He insisted, however, that "more will follow if needed."

NATO estimates Russia has amassed some 40,000 troops on Ukraine's eastern border and could invade parts of the country within days if it wished. Fogh Rasmussen urged Russia to pull those troops back.

The 28-nation alliance has already suspended most cooperation and talks with Russia. The United States has dispatched fighter planes to Poland and the Baltics, enabling NATO to reinforce air patrols on its eastern border. NATO also performs daily AWACs surveillance flights over Poland and Romania.